Strain gage based transducers are used in a variety of applications to convert mechanical inputs (for example, weight, force, mass, torque, pressure, deflection/displacement) into an electrical output. The basis for all such devices is the same. Specifically, a mechanical reaction device (commonly called a spring or counterforce) is designed to respond to the specific input, transducing the input into a measurable surface strain, which changes proportionally with the applied input. Strain gages attached to the transducer counterforce sense and respond to this surface strain with a change in electrical resistance. The counterforce is normally machined from high-quality tool steel (e.g., 4340 or 4140), or highly processed (hardened/heat treated) stainless steel (e.g., 17-4 PH or 17-7 PH), or high-grade, heat treated aluminum (e.g., 2024-T351 or 2024-T81), or other excellent spring materials like beryllium copper or N-Span C. However, there are special cases where polymers are used (e.g., epoxy-glass laminate, or cast/injection molded plastics), and where ceramic materials are used (e.g., Al2O3 99+ percent). In fact, over the course of transducer history, practically every conceivable material has been used at one time or another as the basis for a counterforce. The present invention is not limited to any one material or even to a class of materials; it works well with any material selected for use as a counterforce.
In all cases, strain gage based transducers are used to convert physical loads or inputs into electrical outputs. Achieving the highest level of transducer accuracy requires compensating the device for certain accuracy-limiting effects; some of which are inherent to the strain gage/transducer system, like creep, and some of which are external effects, like changes in temperature, and some of which are a combination, like creep change with temperature, called creep TC. As an example, load cells are used in the weighing industry as transducers to convert a weight (mass/force) into a proportional electrical signal. The load cell is designed mechanically to provide repeatable and quasi-equal-magnitude surface strains at specific points, whereby two of the strains are tensile (positive) and two are compressive (negative). Electrical resistance strain gages bonded at these points convert the surface strains resulting from an applied weight into a proportional electrical signal. The strain gages are connected into an electrical circuit, typically a Wheatstone bridge, which optimizes the output signal.
In the Wheatstone bridge electrical circuit typically used in transducers, four strain gages, plus a power source, are wired together in the series/parallel circuit as depicted in FIG. 2. The electrical nature of this circuit is such that when the bridge is resistively balanced (i.e., all four gages are at nearly the same resistance value) there is no voltage present across the output terminals (O1 and O2). Conversely, when the strain gages are at meaningfully different resistances, there can be a small voltage measured across O1 and O2, proportional to the applied voltage, Vi. Specifically, when gages 1 and 3 increase in electrical resistance and gages 2 and 4 simultaneously decrease in electrical resistance, the maximum proportional output voltage is presented across terminals O1 and O2. It is for this reason that transducer designs incorporate positive and negative strains, so gages bonded at those locations will increase and decrease resistance, respectively, with applied weight; thus, maximizing the voltage signal from the transducer for a given applied weight (maximizing sensitivity).
Within the weighing industry there is a class of load cells used in applications called legal-for-trade. These legal-for-trade load cells must pass stringent qualification tests from internationally recognized standards, such as OIML R60 (Organization Internationale de Metrologie Legal). Results from these tests classify the load cell over a specified temperature range (normally −10 to +40° C.) based upon achievable resolution of weight. The classification metric used is divisions of resolution. For example, a load cell having a maximum combined error of 0.033% is classified as 3000D (3000 divisions) accuracy.
Several factors conspire to affect the classification category of a load cell, including the mechanical design and production of the load cell body, and performance characteristics of the strain gage and its installation. Among the strain gage performance parameters, creep is critical to load cell classification. Ignoring all other error contributions, the allowable cord-slope creep within the example classification (3000D) is 0.0233% FS/min. (percent full-scale per minute).
Transducer creep is defined as a changing output with a stable physical condition or input (weight, in the case of the load cell example) under steady state environmental conditions. Strain gages are custom designed to compensate for the inherent material creep of specific transducer designs. A representative plot of creep for the load cell example is shown in FIG. 3a, which also indicates the chord slope value normally used to quantify the creep, even though the figure clearly shows that creep is a nonlinear phenomenon. Further, creep performance can change, and usually does change using prior creep correction methods, when the transducer temperature is changed from that which was used for initial creep compensation (normally room temperature, T which is ˜24° C.). Changes in creep with temperature can significantly affect the possible classification of a legal-for-trade load cell. The results from creep measurements over a specified temperature range are termed creep TC. In some server application, the temperature range can be T+/−200° C.
Several variables affect strain gage creep, including but not limited to, the resistive material (electrical conductor) from which the strain gage is produced, geometry (e.g., gage length, cross-section dimensions, end loop size, shape, and orientation), construction (materials used in building the gage, including insulating backing and insulating overlay, if present), and installation (thickness and type of cement, gage positioning). The most common type strain gage used in transducers is the thin, metal-foil variety, depicted schematically in FIG. 4. The gage consists of a primary measurement length (gage length), L, a primary measurement width (gage width), Z, a plurality of grid lines, T, configured into a serpentine grid, R, with solder pad connections, M, a plurality of creep controlling end loops, K, an upper alignment guide, P, and a lower alignment guide, N, defining the major measurement axis, J, and an insulating backing (carrier), U.
Prior methods allow for convenient control of transducer creep at room temperature to about 0.0175%/min of rated full-scale output; or, when calculating from OIML R60 for the load cell example, a little over 4000D. One prior method of achieving creep compensation is to select the strain gage end loops (K in FIG. 4) to optimize the creep component. This, of course, presumes proper control of the other previously mentioned effects on creep. This method utilizes four identical, or nearly identical strain gages, with the end loop lengths chosen to provide a chord slope creep as small as possible or, at the least, sufficient for the intended classification. When attempting to achieve the lowest creep slope possible (highest transducer resolution) from a production run of transducers using this prior method, it is typically necessary to grade the production lot, whereby all transducers from the lot are tested and classified by their test results, with no a priori guarantee that any individual transducer from the lot will achieve a high standard.
A subtle variation on the above mentioned prior method of creep compensation is to pick end loop lengths for the strain gages slightly different from one another. With this method, there may be three strain gages with equal end loop lengths and one different; or, two gages with equal end loop lengths and the other two equal, but different from the first two; or, all gages may have a slightly different end loop length. This minor difference of creeping characteristic is achieved using what might be referred to as nearly identical strain gages. This practice primarily evolved from the practical concern over what gages happened to be on-hand when building the transducer, and happen to combine for a low creep result; that is, the method evolved naturally because of inventory practicality. While achieving an excellent creep result at one temperature is possible using the method, it does not, however, necessarily provide any improvement in creep TC performance over the more commonly practiced use of identical strain gages.
Another method of achieving transducer creep compensation has been suggested, whereby the overall stiffness of the strain gage is altered by varying the amount of reinforcing fibers mixed with the backing resin. This method is grounded in the relationship between creep and the relative stiffness difference between the counterforce and the strain gage. One obvious limitation with this technique is its applicability only to mixed-resin backing systems, which is not the dominant type used within the industry.
Achieving high resolution creep compensation over the entire −10 to +40° C. temperature range is a challenging aspect of these prior methods. In another method, various electrical configurations are designed into the strain gage circuit and are formed with the strain gage grids at the time of etching. These configurations are initially electrically inert, but when subsequently introduced into the circuit as active elements by cutting appropriate electrical shunts, the transducer can be creep compensated, including any variation in creeping caused by a change in temperature. This work is performed after the gage has been installed on the transducer. Disadvantages of this method are 1) more complex and costly strain gage design and production; and, 2) careful and selective ‘trimming’ of the creep characteristic in situ.
It is known that strain gage creep, as exhibited by transducer output, is a viscoelastic phenomenon, as illustrated in FIG. 1. As such, when utilizing prior methods of creep compensation, including the method of choosing gages with identical end loops and the method of choosing gages with nearly identical end loops, a typical transducer creep result can be represented by the graph shown in FIG. 1′. Shown in FIG. 5 is a corresponding representative independent strain gage creep from the tension and compression strain gages bonded to a load cell, which combine in the Wheatstone bridge circuit to cause the total transducer creep. From FIG. 5, it is obvious that the direction of creep, as represented by the two curves for change in output with time (one curve for tension strain gages and one for compression strain gages), is opposite in sign; the tension gages are shown decreasing in output (becoming more negative) and the compression gages are shown increasing in output (becoming more positive). As noted previously for the characteristic nature of the Wheatstone bridge, the electrical result from the bridge (the proportional output voltage) of these two opposing creep directions is an increase in that part of the electrical signal caused by creep; subtracting opposite sign signals results in addition of the two signals.
Thus, the prior methods have embraced a common result, where creep compensation is achieved via physical cancelling (viz., the positive counterforce creep is countered by the combined negative tension/compression strain gage creep), but it has not addressed the problem through electrical cancelling as disclosed herein.